1. Field of the Invention
This invention relates to the monitoring of the amount of particulate material, cut tobacco for example, conveyed in a stream thereof. By "monitoring", we mean measuring and/or detecting changes in the amount of material in the stream.
2. Brief Description of the Prior Art
According to the presently conventional mode of making cigarette rod, cut tobacco is fed from a hopper to a conveyor, an endless-band conveyor for example, which conveys it to a garniture in which the tobacco is enwrapped in a continuous web of cigarette paper. Just upstream of the entry end of the garniture, the tobacco encounters a trimmer which serves to remove surplus amounts of tobacco, thus ensuring that the correct amount of tobacco is fed to the garniture. The trimmer may comprise a pair of driven discs, sometimes termed ecreteurs, and a rotary brush for removing the surplus tobacco trimmed off by the discs. It is usual to return the surplus tobacco, known as returns or ecretage, to the hopper.
If the cut tobacco is fed from the hopper to the conveyor at an unduly high rate, the returns per unit time represent a relatively high percentage of the hopper feed per unit time to the conveyor. Experience has shown that as the percentage value of the returns increases, the degree of degradation of the tobacco rises. If, on the other hand, the hopper feed rate is too low, there is a danger that short term variations in the amount of tobacco on the conveyor may make it impossible for the rod weight per unit length to be maintained at a value above a specified minimum value.
Clearly, it is desirable to maintain the percentage returns within present limits about the nominal value. Typically this value is 25%, although other values may be appropriate depending upon the tobacco characteristics and the rod specification.
In United Kingdom Patent Specification No. 958,205, there is disclosed the principle of monitoring the amount of the returns from the trimmer at a location in the path of the returns back to the hopper and, in accordance with signals derived from the monitoring means, varying the speed of a motor driving the hopper feed mechanism. A monitoring means disclosed in the specification comprises a detector flap pivotally mounted a little distance above the surface of a stream of tobacco conveyed in a reciprocating conveyor. The flap rests lightly on the tobacco stream, the downward pressure thereof being adjustable by variation in the position of a counter-weight relative to the pivot point of the flap.
Our experiments have shown that when a detector flap is employed for the purpose of monitoring the amount of tobacco conveyed in a stream, tobacco builds up against the flap at a location upstream of the zone of contact of the flap. This results in inaccurate determination of the amount of, or changes in the amount of, the tobacco being conveyed in the stream. With a view to rectifying this condition, a flap was replaced by a light, freely rotatable, plain roller as the contacting means. It was found that the use of the roller also had the demerit that tobacco built up on the upstream side thereof. Furthermore, there was a tendency for the roller to be thrown violently upwards when it encountered lumps of tobacco at the surface of the stream thereof. Even when a damping device was employed in an attempt to overcome this tendency, the roller did not accurately follow the surface undulations of the tobacco stream and/or caused tobacco build up or blockage.